Seawater is always on the move. Water that comes up to the surface from deep is rich with nutrients. Plankton gather where nutrients are plentiful. Small fish come to eat the plankton. Of course, larger fish and birds come to eat the smaller fish. So upwelling is responsible for an abundance of life.

Deep Currents

Currents also flow deep below the surface of the ocean.
Deep currents
are caused by differences in density at the top and bottom. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense?

Downwelling

Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic.
Downwelling
can take place in other places where surface water becomes very dense (
Figure
below
).

Deep currents flow because of differences in density of ocean water.

When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents.

Upwelling

Sometimes deep ocean water rises to the surface. This is called
upwelling
. The figure below shows why it happens (
Figure
below
). Strong winds blow surface water away from shore. This allows deeper water to flow to the surface and take its place.

An upwelling occurs when deep ocean water rises to the surface.

When water comes up from the deep, it brings a lot of nutrients with it. Why is deep water so full of nutrients? Over time, dead organisms and other organic matter settle to the bottom water and collect. The nutrient-rich water that comes to the surface by upwelling supports many living things.

Upwelling also takes place along the Equator. Winds blow the surface water north and south. This leaves a void that deep water can upwell into. The nutrients rise to the surface and support a great deal of life in the equatorial oceans.

Summary

Cooling or evaporation of fresh water from the sea surface makes surface water dense. This causes the surface water to undergo downwelling.

Downwelling of cold, dense water pushes deep water along. This drives thermohaline circulation.

Upwelling takes place at some coastlines or along the Equator. Upwelling brings cool, nutrient-rich water to the surface.